It is highly desirable for tires to exhibit good traction characteristics on both dry and wet surfaces. However, it has traditionally been very difficult to improve the traction characteristics of a tire without compromising its rolling resistance and tread wear. Low rolling resistance is important because good fuel economy is virtually always an important consideration. Good tread wear is also an important consideration because it is generally the most important factor which determines the life of the tire.
The traction, tread wear and rolling resistance of a tire are dependent to a large extent on the dynamic viscoelastic properties of the elastomers utilized in making the tire tread. In order to reduce the rolling resistance of a tire, rubbers having a high rebound have traditionally been utilized in making the tire's tread. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in the tire's tread. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubber material for automobile tire treads. However, such blends are not totally satisfactory for all purposes.
In some cases, 3,4-polyisoprene can be used in tire tread compounds to improve tire performance characteristics, such as traction. For instance, U.S. Pat. No. 5,104,941 discloses a method of improving the wet skid resistance of a rubber mixture for a tire tread, comprising adding from 5 to 35 parts by weight of a 3,4-polyisoprene to from 95 to 65 parts by weight of a sulfur-vulcanizable elastomer containing conventional fillers, oils, auxiliaries and vulcanizing agents, wherein said 3,4-polyisoprene is at least partially incompatible with said sulfur-vulcanizable elastomer and has (a) a 3,4-content from 55 to 75 percent, as determined by NMR spectroscopy, (b) a glass transition temperature from 0.degree. to -25.degree. C., determined by differential scanning calorimetry at a heating rate of 10.degree. C./minute, (c) a number average molecular weight Mn, determined by gel permeation chromatography of 220,000 or higher, and (d) an inhomogeneity U of less than 1.8, the inhomogeneity being defined by the equation U=Mw/Mn-1, where Mw and Mn are determined by gel permeation chromatography. U.S. Pat. No. 5,104,941 further discloses a method for improving the wet skid resistance of a rubber mixture, comprising adding from 5 to 35 parts by weight of a 3,4-polyisoprene to from 95 to 65 parts by weight of a sulfur-vulcanizable elastomer, further containing conventional fillers, oils, auxiliaries or vulcanizing agents, wherein said 3,4-polyisoprene is at least partially incompatible with said sulfur-vulcanizable elastomer and has (a) a 3,4-content of from 55 to 75 percent, determined by NMR spectroscopy, (b) a glass transition temperature from 0.degree. to -25.degree. C., determined by differential scanning colorimetry (calorimetry) at a heating rate of 10.degree. C./minute, (c) a number average molecular weight, determined by gel permeation chromatography, of from 200,000 to 218,000, and (d) an inhomogeneity U of 1.4 or less.
U.S. Pat. No. 5,087,668 and U.S. Pat. No. 5,300,577 disclose a pneumatic tire having an outer circumferential tread where said tread is a sulfur-cured rubber composition composed of, based on 100 parts by weight rubber, (a) about 5 to about 35, preferably about 10 to about 25 parts by weight 3,4-polyisoprene rubber, (b) about 20 to about 60, preferably about 30 to about 55 parts by weight cis 1,4-polyisoprene rubber, and (c) about 10 to about 50 parts by weight of at least one other rubber selected from at least one of solution polymerization formed styrene/butadiene copolymer rubber having a styrene/butadiene ratio in the range of about 5/95 to about 30/70, preferably about 8/92 to about 25/75, emulsion polymerization formed styrene/butadiene copolymer rubber having a styrene/butadiene ratio in the range of about 10/90 to about 60/40, preferably about 15/85 to about 35/65, cis 1,4-polybutadiene rubber, isoprene/butadiene copolymer rubber having an isoprene/butadiene ratio in a range of about 30/70 to about 70/30, styrene/isoprene rubber having a styrene/isoprene ratio in a range of about 10/90 to about 35/65 and styrene/isoprene/butadiene rubber; wherein said 3,4-polyisoprene rubber, in its uncured state, is characterized by having a glass transition temperature (Tg) in the range of about -15.degree. C. to about -20.degree. C. a Mooney (ML1+4) value in the range of about 70 to about 90, preferably about 75 to about 85, and, further, a polymer structure containing about 40 to about 70 percent, preferably about 50 to about 60 percent, 3,4-polyisoprene units, about 30 to about 50 percent 1,4-cis and trans units and about 2 to about 10 percent 1,2-polyisoprene units with the total of its 3,4 and 1,2 units being in the range of about 56 to about 63 percent.
U.S. Pat. No. 5,239,023 and U.S. Pat. No. 5,151,398 disclose a process for the synthesis of 3,4-polyisoprene which comprises polymerizing isoprene monomer in an organic solvent at a temperature which is within the range of about -10.degree. C. to about 100.degree. C. in the presence of a catalyst system which is comprised of (a) an organoiron compound which is soluble in the organic solvent, wherein the iron in the organoiron compound is in the +3 oxidation state, (b) a partially hydrolyzed organoaluminum compound which was prepared by adding a protonic compound selected from the group consisting of water, alcohols and carboxylic acids to the organoaluminum compound and (c) a chelating aromatic amine; wherein the molar ratio of the chelating amine to the organoiron compound is within the range of about 0.1:1 to about 1:1, wherein the molar ratio of the organoaluminum compound to the organoiron compound is within the range of about 5:1 to about 200:1, and wherein the molar ratio of the protonic compound to the organoaluminum compound is within the range of about 0.001:1 to about 0.2:1.
U.S. Pat. Nos. 5,231,153, 5,336,739 and 5,448,003 disclose a process for the synthesis of 3,4-polyisoprene which comprises polymerizing isoprene monomer in an organic solvent in the presence of a catalyst system which is comprised of (a) a lithium initiator and (b) an alkyltetrahydrofurfuryl ether modifier, wherein the alkyl group in the alkyltetrahydrofurfuryl ether modifier contains from 6 to about 10 carbon atoms.
U.S. Pat. No. 5,534,592, discloses an initiator system which is comprised of (a) a lithium initiator, (b) a sodium alkoxide and (c) a polar modifier; wherein the molar ratio of the sodium alkoxide to the polar modifier is within the range of about 0.1:1 to about 10:1; and wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.01:1 to about 20:1.